Recently, a so-called "instant video portrait system" has been known, which is a system utilizing a sublimation transfer recording technique. This system operates by imaging an object with a video camera, then thermally transferring the video picture information as a static image onto a dye receptor layer of printing paper with the sublimation transfer recording technique, and using the resulting transfer image for various certificates. Thus, this system has been noted as a substitute of the conventional silver salt photography. For instance, this "instant video portrait system" has been adapted in Europe for specifying users of a ski pass or a railroad pass.
Such a thermal transfer image formed on the printing paper may be used in the exposed state for a variety of purposes. However, it is a prevalent practice to stack a cover film for thermal transfer image on the thermal transfer image for protecting it. The cover film for thermal transfer image can be exemplified by a polyester film having an adhesive layer formed thereon or a polyvinyl chloride film which can be stacked by thermo-compression bonding of itself.
In some cases, a water-color ink image, such as an overlapping seal or various stamped images, may be formed by using a water-color ink on the thermal transfer image formed on the printing paper.
However, since the dye receptor layer of the printing paper in which the thermal transfer image is formed is made of a lipophilic or water repellent material, such as polyester, the dye receptor layer does not absorb or is not penetrated by the water-color ink. Therefore, the dye receptor layer does not fix the water-color ink image therein.
Also, since the conventional thermal transfer image cover film is made of a lipophilic material as described above, the cover film does not absorb or is not penetrated by the water-color ink, and therefore does not fix the water-color ink image therein. Thus, the water-color ink image formed on the thermal transfer image is easily destroyed by any contact with water dropping due to rain or perspiration or by rubbing by hand.
Although it is conceivable to form a cover film for thermal transfer image only of a water soluble material to solve the above-described problems, stacking such a cover film directly on the lipophilic receptor layer is extremely difficult and lacks water resistance.